1. Industrial Field
The present invention relates to a reflection type liquid crystal display for performing a display by reflecting incident light from outside and to a method for manufacturing the same. The invention also relates to a mask for manufacturing such a reflection type liquid crystal display.
2. Prior Art
Research and development have been aggressively conducted on a liquid crystal display serving as a flat panel display taking the place of a CRT display. In particular, a liquid crystal display characterized by consuming less electric power and having a thin configuration has been put into practical use in the field of a battery-driven small-sized television set, a notebook-type computer, a car-navigation system, a portable terminal apparatus, and so on.
An active-matrix-type TFT array utilizing a thin film transistor (hereinafter described as TFT) as a switching element is generally used as a driving method of the liquid crystal display because of high-quality display. The display has a transmission type construction or a reflection type construction. Unlike the transmission type, the reflection type does not require any back light source and consumes less electric power, and is extremely suitable for a portable terminal apparatus and so on. This reflection type liquid crystal display comprises a first insulating substrate having scanning lines and signal lines formed into a lattice configuration, a TFT, a reflex picture element electrode and so on, and a second insulating substrate having a color filter, a black matrix, an opposed electrode and so on. The first insulating substrate and the second insulating substrate are opposed to each other. A liquid crystal is placed between these substrates.
Display quality of the reflection type liquid crystal display is effectively improved by increasing an effective display area in a picture element portion of a liquid crystal display panel and increasing light utilization efficiency, i.e. increasing picture element aperture efficiency. It is said that a TFT array of high aperture efficiency picture element is effectively obtained by the steps of: forming an interlayer insulating film comprising insulating resin of a thickness enough to dissolve difference in level caused by the scanning lines, signal lines and TFT; forming a picture element electrode overlapping with the foregoing scanning lines and signal lines occupying a large area on the interlayer insulating film; and connecting the picture element electrode to a drain electrode of the TFT through a contact hole in the interlayer insulating film. In this method, it is possible to prevent a defect caused by unevenness on the substrate in rubbing process.
On the other hand, to increase light utilization efficiency, a method of using a picture element electrode combined with (or serving also as) a reflex film has been proposed, wherein the mentioned first insulating substrate receives a scattering light having good directivity without applying any scattering film on the incident light side (i.e., without adopting forward scattering plate method). In this method, a good scattering light is obtained by forming appropriate unevenness on the surface of the picture element electrode combined with the reflex film. A reflection type liquid crystal display, wherein this construction is adopted and unevenness is formed on the surface of the photosensitive insulating resin by a photolithography, is disclosed in the Japanese Patent Publication (unexamined) No. 90426/1997.
The conventional reflection type liquid crystal display comprises: a first substrate (a TFT array substrate) having a TFT including a matrix-shaped scanning electrode, a signal electrode, a semiconductor layer and so on, and a reflection type picture element electrode formed on a transparent insulating substrate, and electrode wiring formed around the picture element electrode; a second substrate (an opposed substrate) having a color filter, a black matrix (hereinafter described as BM), and an opposed electrode on another transparent insulating substrate opposed and adhered to each other; and a liquid crystal material injected in between the first substrate and the second substrate.
Display quality of the reflection type liquid crystal display is effectively improved by increasing the effective display area in the picture element portion of the liquid crystal display panel and increasing the light utilization efficiency, i.e. by increasing the picture element aperture efficiency. In order to obtain a TFT array of high aperture efficiency, a method has been proposed, in which after forming a TFT and electrode wiring on a transparent insulating substrate, a flat surface is formed thereon by forming an interlayer insulating film composed of a resin to coat them. Further, a reflex picture element electrode occupying a large area is formed on the interlayer insulating film in such a manner as to overlap with a scanning electrode and so on laid under the interlayer insulating film. The reflex picture element electrode and a drain electrode of the TFT are electrically connected through a contact hole formed in the interlayer insulating film.
As a method for further increasing the light utilization efficiency, a reflection type liquid crystal display has been proposed. In this method, a TFT array substrate is provided with a picture element electrode combined with a reflex film for obtaining a scattering light of good directivity, without adopting any forward scattering plate method for applying a scattering film on the incident light side. In such a reflection type liquid crystal display, a good scattering light is obtained by forming an appropriate unevenness on the surface of the picture element electrode combined with the reflex film.
The Japanese Patent Publications (unexamined) No. 175126/1994 and No. 184846/1996 disclose a reflection type liquid crystal display having a picture element electrode combined with the reflex film. In this liquid crystal display, minute unevenness on the surface of the resin forming the interlayer insulating film is formed by photolithography using a photosensitive resin (including resist and so on), and the picture element electrode consisting of a film having a high reflection characteristic is formed thereon. Thus, unevenness is formed on the surface of the picture element electrode.
The foregoing Japanese Patent Publication (unexamined) No. 90426/1997 discloses a method for forming simultaneously an unevenness pattern and a contact hole pattern by using a mask in which both unevenness pattern and contact hole pattern are formed and by changing dissolution velocity in development utilizing difference in size between them. It is, however, very difficult to form the contact hole and obtain stably the unevenness for the reflex film capable of obtaining a good scattering light on the resin surface at the same time. Moreover, in order to obtain a good scattering light without mirror reflection, it is necessary for the unevenness pattern to have a certain size. Since the dissolution velocity of the unevenness pattern is almost the same as that of a contact hole pattern, it is very difficult to form the unevenness pattern and the contact hole pattern with distinction between them.
Moreover, the Japanese Patent Publication (unexamined) 198919/1995 discloses a method for forming a reflex plate having unevenness on the surface by using an exposure mask of which transmission amount of light is controlled and by performing an exposure while changing the amount of light in multiple stages in a depth direction of a photosensitive film. However, it is necessary that there is not any flat portion in order to obtain a good scattering characteristic, and for example, approximately 200 to 300 uneven portions are required in one picture element in the case of a 12. 1SVGA array. With respect to 1,440,000 picture elements in all, it is necessary that each unevenness configuration between one picture element and another is uniform in order that there is no reflection spot. Consequently, a mask capable of conducting exposure satisfying such requirements becomes very expensive, and it is very difficult to manufacture such a mask. Moreover, the resin exposed and developed is treated with heat, whereby the resin is turned into fluid due to this heat treatment and is formed into a specific configuration defined according to the physical property value of the resin. A problem exists in that even when unevenness is formed changing the exposure amount in multiple stages, neighboring minute uneven portions disappear.
The invention was made to solve the above-discussed problems.
Furthermore, it is an object of the present invention to provide a method for manufacturing a reflection type liquid crystal display wherein a TFT array substrate of high aperture efficiency driven with a low electric power and having a superior display quality is stably obtained through a simple process.
More specifically, a method for manufacturing the reflection type liquid crystal display by forming minute unevenness on the surface of the picture element electrode combined with the reflex film by a photolithography has been proposed in order to increase the light utilization efficiency. In this method, however, when photosensitive resin used for forming the unevenness is exposed, ultraviolet light not absorbed by the photosensitive resin portion transmits through the transparent substrate, and a part of the unabsorbed ultraviolet light reflects at a substrate holder. In particular, the ultraviolet light reflected at an end portion of an absorption groove, etc. provided on the substrate holder becomes stray light which exposes the photosensitive resin near the absorption groove. This brings about minute difference in size of the unevenness configuration obtained in a region corresponding to the configuration of the absorption groove. In the case of forming a high reflex film on a film where such unevenness is formed, the unevenness portion having minute difference in size is also transferred to the high reflex film as it is. And such a portion is visually recognized as a reflection spot and picture quality of the display is eventually deteriorated.